The present invention relates in general to optical wireless communications and, more particularly, to apparatus and methods for establishing and maintaining a reliable optical wireless data link between to transmitting and receiving units.
Modern data communications technologies have greatly expanded the ability to communicate large amounts of data over many types of communications facilities. This explosion in communications capability not only permits the communication of large databases, but has also enabled the digital communication of audio and video content. This multimedia communication requires high bandwidth communication, which is now carried out over a variety of facilities, including telephone lines (e.g., fiber optic and twisted pair), coaxial cable (e.g., as supported by cable television service providers), dedicated network cabling within an office or home location, satellite links, and wireless telephony.
Each of these conventional communications facilities involves certain limitations in their deployment. In the case of communications over the telephone network, high-speed data transmission, such as that provided by digital subscriber line (DSL) services, must be carried out at a specific frequency range to not interfere with voice traffic, and is currently limited in the distance that such high-frequency communications can travel. Of course, communications over xe2x80x9cwiredxe2x80x9d networks, including the telephone network, cable network or dedicated network, requires the running of the physical wires among the locations to be served. This physical installation and maintenance is costly, as well as limiting to the user of the communications network.
Wireless communication facilities of course overcome the limitation of physical wires and cabling, and provide great flexibility to the user. Conventional wireless technologies involve their own limitations, however. For example, in the case of wireless telephony, the frequencies at which communications may be carried out are regulated and controlled. Furthermore, current wireless telephone communication of large data blocks, such as video, is prohibitively expensive, considering the per-unit-time charges for wireless services. Additionally, wireless telephone communications are subject to interference among the various users within a nearby area. Radio frequency data communication must be carried out within specified frequencies, and is also vulnerable to interference from other transmissions. Satellite transmission is also currently expensive, particularly for bidirectional communications (i.e., beyond the passive reception of television programming).
Recently, attention has turned to optical wireless networking for data communications. Using this technology, data is transmitted by modulating a light beam, in much the same manner as in the case of fiber optic telephone communications. A photo-receiver receives the modulated light, and demodulates the signal to retrieve the data. As opposed to fiber optic-based optical communications, however, this approach does not use a physical wire for transmission of the light signal. In the case of directed optical communications, a line-of-sight relationship between the transmitter and the receiver permits a modulated light beam, such as that produced by a laser, to travel without the waveguide of a fiber optic cable.
Hence, optical wireless networks could provide numerous important advantages over other conventional communications systems. First, high frequency light modulation can provide for high bandwidth data communication (e.g., xcx9c100 Mbpsxe2x80x94Gbps). This high bandwidth need not be shared among multiple users, especially when carried out over line-of-sight optical communications between transmitters and receivers. Without other users on the communications link, of course, the bandwidth is not limited by interference from other users, as in the case of wireless telephony. Modulation can also be quite simple, as compared with multiple-user communications that require time or code multiplexing of multiple communications signals. Bi-directional communication can also be readily implemented utilizing this technology. Furthermore, optical frequencies are not currently regulated, and as such no licensing is required for the deployment of extra-premises networks.
These attributes of optical wireless networks make this technology attractive both for local networks within a building, and also for external networks. Indeed, it is contemplated that optical wireless communications may be useful in data communication within a room, such as for communicating video signals from a computer to a display device, such as a video projector. The costs and effort associated with routing and placing cables in congested, space constrained areas can be eliminated using optical wireless links. If reliable enough, modems using optical wireless links would be especially valuable in mobile product devices such as laptop computers and handheld organizers.
A common problem with some conventional optical wireless links, however, is that they utilize relatively wide, diffuse optical beams to facilitate the acquisition and maintenance of a light link. The ability to correctly aim a transmitted light beam at a receiver is of importance in optical communications technology. Wider beams can allow for greater tracking tolerance, because exact positioning of a transmitting beam on a receiver is not required to maintain a nominal communication link. The use of wider beams, however, either decreases the intensity (i.e., power) of the beam at the receiver or increases the power required to deliver a high data rate signal, and can result in severe limitations in the usable bandwidth of the data link(s) established, thus decreasing the usefulness of link for many communication applications.
Some conventional systems attempt to use narrower, more tightly focused optical beams (e.g., laser generated collimated beams) to provide greater communications bandwidth. When utilizing laser-generated collimated beams, which can have quite small spot sizes, the reliability and signal-to-noise ratio of the transmitted signal are degraded if the aim of the transmitting beam strays from an optimum point at the receiver. Considering that many contemplated applications of this technology are in connection with equipment that will not be precisely located, or that may move over time, it is necessary to be able to rapidly and reliably adjust the aim of the light beam.
Because the integrity of communications does rely on precise optical alignment, conventional solutions can also present problems in circumstances where transceiver units are subject to some vibration or sway (e.g., a building to building link, or a mobile to stationary link). Many conventional systems rely on a low bandwidth direct feedback channel between transceivers, such as a secondary telephone line modem, and some gross mechanical adjustment (e.g., a motorized mechanical assembly housing one or more of the transceivers) to maintain transceiver alignment. Such conventional systems can have problems responding when high frequency vibrations occur, and make it difficult, if not impossible, to successfully track and maintain communications with a moving transceiver. Finally, such conventional systems are often not able to translate changes in signal strength, which is a common method of measuring the integrity of a communications link, into usable positioning information for the mechanical assembly.
Thus, when either a high degree of transceiver mobility is required, or when transceivers may be subject to high frequency or small scale vibrations, conventional systems are typically incapable of providing reliable, high bandwidth communication.
Therefore, a versatile system for acquiring and maintaining reliable optical wireless links that provides for simple and cost-effective high performance optical communications, especially where fixed optical units are subject to high frequency vibrations or where optical units are in motion relative to one another, is now needed, providing for efficient and practical utilization of optical wireless communications in mobile products and devices while overcoming the aforementioned limitations of conventional methods.
The present invention provides a system for implementing an optical communications network. The present invention determines optical beam position information with respect to time at a receiver of an optical wireless link unit. The optical beam is transmitted from a second optical wireless link unit in response to a predetermined beam steering input. The relative motion of the units in relation to one another, and with respect to time, will result yield beam position profiles over time. A beam steering element effectively separates the motion into two components. The first component corresponds directly to the beam steering input, which is predetermined. The second component corresponds to the relative motion, which can be of variable frequency or amplitude. A high bandwidth return channel is provided to relay a high resolution portrait of the beam location profile over time. The present invention processes and utilizes this information to adjust the beam steering element, correcting for the motion or vibrations and maintaining the optical data link between the units. The present invention thus provides robust and efficient optical wireless communications within a given fixed or mobile network or system.